Due to growing environmental awareness and increasing regulation governing the emission from exhaust stacks, particularly of power generating plants using coal-fired burning facilities, numerous efforts have been made to firstly detect the various components of a stack gas stream and to secondly address the transformation of some of these components into gases having a reduced adverse environmental impact. With respect to detecting individual component gases in a stack gas stream, spectrophotometric approaches have been developed since spectrophotometric analysis of a plurality of gases provides, at least in principle, for relatively precise determination of individual gases in a stack gas stream, with each of the component gases having in its pure or isolated from a characteristic absorption spectrum. Thus, it has been possible to detect with suitably designed spectrophotometers a concentration of a trace amount of one gas present in a larger amount of another gas, where the other gas has a characteristic absorption spectrum which substantially overlaps a characteristic absorption spectrum of the trace gas.
The design and operation of spectrophotometers per se, and of spectrophotometers adapted for use in gas analysis, are well known. Such known spectrophotometers, also referred to as spectrometers or spectral analyzers, will be described briefly hereinafter in conjunction with considerations of requirements for providing gas samples representative of the stack gases in an operative environment of a power plant such that the samples can be characterized spectrophotometrically under conditions which most closely resemble the composition of the gases in the stack. ideally, in such an environment of an operating plant, various conduits and associated valves, flow regulators, and a gas flow-through chamber for optical analysis of stack gas samples would be provided during the initial construction of the plant or would be provided during an upgrade thereof. A spectrophotometer, preferably of a readily transportable type without moving parts therein, could then be brought to each plant so equipped, optically coupled into the pre-installed flow-through chamber, and deployed to determine concentration levels of gases to be monitored on a periodic basis, for example on a monthly basis.
A stack gas stream, particularly a stack gas stream from a coal-fired facility, typically includes numerous gases, such as, for example, CO, CO.sub.2, NO, NO.sub.2, SO.sub.2, and water vapor, with each of these gases having substantially differing levels of concentration and having differing characteristic absorption spectra. Additionally, stack gases entering a stack frequently are at a temperature of approximately 300.degree. C.
When it is necessary to reduce the concentrations of the nitrogen oxide gases (NO, NO.sub.2) in the stack gas stream, a reactive gas, for example, NH.sub.3, is added to the stack gas stream at or near the base of the stack so as to react with the nitrogen oxide constituents of the stack gas and thereby forming through a chemical reaction molecular nitrogen (N.sub.2) and water vapor, such transformed compounds being environmentally significantly more benign than the nitrogen oxides and the NH.sub.3 addition gas. It has been found in the operation of fossil fuel-fired facilities that it is advantageous to add a slight excess of the NH.sub.3 reactive gas over the amount required to transform the nitrogen oxides into molecular nitrogen and water vapor and to spectrophotometrically monitor the concentration of this slight excess of NH.sub.3 in a sample of the remaining stack gases, rather than to monitor the concentration of the molecular nitrogen and water vapor.
While NH.sub.3 has a characteristic absorption spectrum in an ultraviolet spectral region of wavelengths, so does unfortunately the SO.sub.2 constituent of the stack gas stream. Since the concentration of SO.sub.2 in the stack gas stream can be in the range of 2,000 parts per million (ppm) depending on the type of fuel being burned, the concentration of the residual or trace quantity of NH.sub.3 being kept to a minimum level frequently in the range of from 2-10 ppm. Accordingly, at a selected gas flow rate of a sample through a gas flow-through chamber having particular dimensions, a spectrophotometric measurement in an ultraviolet spectral region can, for example, indicate an absorbance value of 1.0 for SO.sub.2, and an absorbance value of 0.10 or less for the residual NH.sub.3 constituent, with both measurements performed at a particular wavelength, for example, at a wavelength of 200 nm. Thus, in order to reliably detect low concentrations of NH.sub.3 in a gas stream having a plurality of gases, including SO.sub.2 at relatively high concentration levels, it becomes imperative to devise a stack gas monitor system which provides the capability to conduct such measurements at a periodic basis with relative ease in the operating environment of an industrial or commercial facility.
It is also known that when the temperature of the flow-through chamber or measuring chamber and the conduits leading to and from the chamber is allowed to decrease to a temperature lower than about 250.degree. C., a chemical reaction between the SO.sub.2 and the NH.sub.3 can commence, thereby forming an ammonium sulfate deposit, and more specifically an ammonium bisulfate deposit, within the conduits and the measuring chamber. Such formation of ammonium sulfate not only reduces the measured concentration of ammonium gas (NH.sub.3) in the flow-through chamber by that reaction, but can also lead to a relatively rapid buildup of deposits within the inner walls of the conduits and within the measuring chamber thereby necessitating either time-consuming and expensive cleanout procedures during which the gas flow of the sample gas stream has to be suspended, or in a worst case, requiring the operation of the burner facility of the plant to be disrupted. Accordingly, it is important to prevent or at least to minimize the buildup of such ammonium sulfate deposits by ensuring that the temperature of at least a conduit extending between the stack and a flow-through measuring chamber, the temperature of the chamber, and preferably the temperature of a conduit extending between the chamber and a gas flow monitor system is maintained at about 300.degree. C., which is a temperature comparable to the temperature of the stack gas stream entering the stack.
When it is required to reliably detect a concentration of a first gas which is present at relatively low concentration in a second gas, and both gases have substantially overlapping characteristic absorption spectra, it has been recognized in the art of spectrophotometric analysis of gas mixtures that a gas concentration calibration is desirable for at least the one gas having the lower or trace concentration and also having lower absorbance values compared to the second gas over the same spectral region.
It has also been appreciated in the art of spectrophotometric gas analysis to establish a spectrophotometric reference level which can be a so-called blanking level corresponding to a spectrophotometric determination of the spectral absorbance of a measuring chamber in either the absence of any absorbing gas, such as would prevail if the chamber were to be evacuated, or a reference or blanking level established by flowing a gas through the chamber whereby the flowing gas exhibits no absorbance in the spectral region of interest with respect to components of the stack gases to be investigated.
Keeping in mind the above considerations, major features of the art pertaining to spectrophotometric determination of the concentration of at least one gas in a plurality of gases, and known to the applicant, will be described briefly in the following:
In U.S. Pat. No. 4,126,396 there is disclosed a method and apparatus for non-dispersive optical determination of gas and smoke components in a mixture of stack gases by reflection and detection of radiation having a different wavelength for each gas component, and wherein the radiation is periodically intercepted before it enters the gas mixture and is reflected back to a photoreceiver. The system uses a rotating filter wheel disposed in front of the photoreceiver to select in succession three different and distinct wavelengths of radiation dedicated to the measurement of smoke components, and of the gas components for the gases SO.sub.2 and NO.sub.2, respectively. Reference signals are provided at each of the test wavelengths by selectable insertion of a reference reflector in the radiation beam.
In U.S. Pat. No. 3,979,589 there is described a method and system for the infrared analysis of gases which is particularly directed at detecting automobile emissions. In a preferred single path instrument a rotating filter wheel is used between a sample gas cell and a detector to sequentially select a first wavelength at which a gas in the cell appears absorptive, and a second wavelength of infrared radiation at which the gas in the measuring cell appears non-absorptive, thereby establishing a reference signal level. The infrared gas analyzer is calibrated by a zeroing procedure which utilizes ordinary air.
In U.S. Pat. No. 5,053,623 there is disclosed a photometric analyzer for measuring differential total reduced sulfur in a sample gas stream. A sample measuring cell alternately receives a gas sample of SO.sub.2 gas from a SO.sub.2 sample supply, and a sample of SO.sub.2 gas which was converted in an oven from the total reduced sulfur. Additionally, zero air is supplied to the sample cell for measurement by the analyzer of the SO.sub.2 levels in the sample gas. The measurements are performed at a single wavelength of radiation of about 300 nm.
In U.S. Pat. No. 4,441,815 there is disclosed a wavelength-modulated derivative spectrometer for continuous measurement of trace amounts of NH.sub.3 gas in a stack gas which includes a high concentration of SO.sub.2. The system has features which can substantially eliminate interference from the spectrum of SO.sub.2 which has a relatively finely structured periodic absorption spectrum so as to enable measurement of small concentrations of NH.sub.3 which has a relatively broad structure absorption spectrum. The self-modulating spectrometer has an entrance slit, a dispersion grating, and an exit slit, coupled by suitably disposed reflective elements. Wavelength modulation is provided by periodically scanning the wavelength of light passed out from the exit slit by means for vibrating either the entrance slit or the exit slit or by vibrating other components of the instrument. The above reference also discloses probe means for extracting a gas sample from a stack gas stream, and heated conduit means, as well as a heated measuring cell so as to prevent a possible chemical reaction between the NH.sub.3 and the SO.sub.2 gas and condensation of water vapor in the sample gas stream.
In U.S. Pat. No. 5,272,345 there is disclosed a calibration method and apparatus for measuring the concentration of components of a fluid, in which for example, the concentration of NH.sub.3 gas can be determined spectrophotometrically in the presence of an interfering gas such as sulfur dioxide (SO.sub.2) or nitric oxide (NO). The spectrophotometric measurements are determined from an intensity of radiation over a selected range of wavelengths using a peak-to-trough calculation. A reference level is established by measuring the amount of radiation which passes through a non-absorptive zero gas (such as N.sub.2), or which alternatively passes through an evacuated measuring chamber. The disclosed apparatus uses an optical grating and a photodiode array detector to generate signals corresponding to radiation passing through the sample gases to be measured, and of signals corresponding to signals of the zero gas or evacuated chamber condition. Referring to an article by Nakabayashi et al, titled "Development of Low Level NH.sub.3 Measuring Method," the above disclosure appears to indicate that it is undesirable or unnecessary to maintain certain conduits and the measuring chamber of a stack gas monitoring system at an elevated temperature.
In a European Patent Application No. EP0591758, there is disclosed a multiple component analysis arrangement for detecting gases or liquids, in which a pulsed radiation source is provided to direct ultraviolet radiation pulses to a measuring cell through which the sample gases or liquids are flowing, and providing through a beam-splitting arrangement a reference beam of pulses which is used to determine the stability of the pulsed radiation source. The arrangement uses a spectrophotometer of the type having a diffraction grating for creating a spectral dispersion of light incident thereon and for directing the dispersed light onto a linear photodiode array for simultaneous measurement at a plurality of wavelength. The arrangement relies on establishing a baseline or blank spectrum in which non-absorptive gases such as air or nitrogen are used in the measuring chamber to provide for a baseline calibration of the optical components of the spectrometer. Such baseline calibration data are stored in a storage element of a data processing system associated with the instrument. The above publication suggests the use of a massive quartz rod for coupling radiation out of the radiation source and a separate quartz rod for coupling radiation into an entrance slit of the spectrophotometer. Not suggested in this publication are particular approaches to calibration of the analysis arrangement with respect to the gases or liquids to be analyzed, nor is there any teaching on how to protect the measuring cell and its inlet and outlet conduits from contamination due to deposits arising from the measurement of emission products from stacks of fossil fuel burning facilities.
Thus, while each of the aforementioned publications appears to offer a particular solution to multi-component gas analysis, none of the above approaches appear to incorporate all those features which would be desirable in a stack gas monitor system for real-time spectrophotometric determination of a concentration of a first gas which can be selectably introduced as a measured amount into a stack gas stream of a plurality of stack gases, where the plurality of stack gases would serve as a blanking or reference level in a spectrophotometric system in the absence of the first gas.